Justification:Neritina granosa has been assessed as Vulnerable under criterion B1ab(i,ii,iii,iv) due to its restricted distribution. It has an estimated extent of occurrence of less than 20,000 km2, a severely fragmented distribution, is unlikely to occur in more than ten locations (based on current knowledge of stream occupancy) and shows a decline in habitat quality, number of locations, and AOO. It is extirpated from some islands. If it were possible to estimate population declines (or use a proxy), then this species may qualify for a higher threat category.

This species is found in high quality streams on four Hawaiian islands. It was historically on all the islands of Kaui, Maui, Molokai, Hawaii and Oahu, restricted to high-quality streams (Hawaii Biological Survey 2010). It is likely now to be present in around six to ten locations, based on known stream occurrences (see population section), and is only confirmed from four of the Hawaiian islands, Maui, Hawaii, Kaui, and Oahu (Hodges 1992, Hodges and Allendorf 1998, Fitzsimons et al. 2005, Hau 2007, Hawaii Watershed Atlas 2008, Kido 2008, Hawaii Biological Survey 2010). Its estimated extent of occurrence is less than 20,000 km2.

This species was formerly common in Hawaii, but has declined throughout its range and is now present in only a few relatively pristine streams (Brooks 2009).

Maui: Hodges (1992) and Hodges and Allendorf (1998) reported populations ranging from 70,000 to 350,000 individuals in three streams on Maui and many other stream populations probably were of a similar size, although some may have been larger. Monthly counts were from 576 in Ïao Stream (1999-2004) while Honomanü Stream counts were from 67912 (Hau 2007). Known number of locations is at least three (three streams), but may be more.

Hawaii: Recent surveys on Kawainui stream, Hawaii island, found this species in densities of 9.5 ind m-2 in the middle part of the stream (Hawaii Watershed Atlas 2008). Known number of locations is at least one (Kawainui stream), but may be more.

Kaui: In a stream on Kaui, this species extended from the stream mouth to 259 m asl (although it has been recorded up to 400 m asl), being most abundant in the lower stretch. Densities ranged from 0.01 to 0.02 ind. per sq. m (Kido 2008). Known number of locations is at least one (one stream on Kaui), but may be more.

Oahu: Here, the species is uncommon (Hawaii Biological Survey 2010). A recent survey on a stream in Oahu recorded only two individuals (Fitzsimons et al. 2005). Known number of locations is at least one (one stream on Oahu).

This amphidromous species (migrates between salt and freshwater for reproduction) inhabits relatively pristine, fast-flowing streams with boulders and gravel substrates (Hawaii Biological Survey 2010). Adults feed on algae in the benthic zone (Brooks 2009). It may also occur under waterfalls (Pyron and Covich 2003).

A variety of threats have caused past declines in this species. These include: impoundment; water diversion and changes in stream flow; urbanisation; and habitat alteration or shading brought about by non-native riparian vegetation (Brooks 2009). Further threats that may impact this species include siltation via erosion of disturbed watersheds, direct or indirect pollution of surface or groundwater, and introduction of invasive species that may predate on or compete with native species (Gon III 2006).

Impediments to natural flow and saline water encroachment are the greatest threats, however natural flow restoration is possible (Hau 2007). Such diversions in Iao and Honomanu Streams on the island of Maui have reduced stream flow, and with it the reproductive success of this species (Hau 2007). This means the snails are unable to complete their migration upstream and are restricted to estuaries, where they are vulnerable to tidal action and desiccation (Hau 2007). They are limited in their upstream migration by low stream flows and instream obstructions (HI NHP pers. comm. 2008).

Amphidromous species are also vulnerable to deterioration in more than one habitat, such as freshwater, estuaries and the open sea (Hodges and Allendorf 1998). Reduced stream quality on Oahu island has meant that populations there have declined (Hawaii Biological Survey 2010). This large snail is also an important food item for Native Hawaiians (Hodges and Allendorf 1998). Flooding also results in overflowing sewage plants entering streams (Goodwin 2006).

Extensive flooding - resulting in high sedimentation, turbidity, nutrients, trash and other pollutants emptying into streams, rivers, estuaries, and oceans is also a major threat. The recent flooding event caused wastewater treatment plants to overflow - causing hundreds of thousands of gallons of raw and untreated sewage to enter streams inhabited by this species (Goodwin 2006).

This species has a Global Heritage Status of G1 - Critically Imperiled (NatureServe 2009). Few populations are protected, although restocking efforts are underway in some parts of its range (Goodwin 2006).

Surveys to establish population size, distribution and trends on all islands in the Hawaiian Archipelago are recommended. Stream restoration, management and protection are vital to ensure continued viability of most populations of this species.